This invention relates to a local overheating diagnosis method and apparatus for a rotary electric machine, and more particularly to a diagnostic method and apparatus for local overheating occurring in the coil, core, or the like of a rotary electric machine such as a gas cooled turbo-generator through diagnosis of the concentration in cooling gas of submicron particles generated from varnish coated on the surface of the relevant overheated portion.
As the capacity of each unit of a turbo-generator increases, the deterioration of the insulation coating of the generator may invite a dangerous failure, and especially a high possibility of causing a burning failure around the core or coil of the generator. There have been proposed a number of diagnosis systems for detecting local overheating thereby to prevent a burning failure by detecting submicron particles generated from varnish insulator coated on the surface of the coils or cores and estimating the degree of the temperature rise of the relevant place.
One example of such a prior system is U.S. Pat. No. 3,427,880 issued Feb. 18, 1969 to Lloyd P. Grobal et al, entitled "Overheating detector for gas cooled electric machine". According to this patent, the local overheating diagnosis is based on the fact that when the insulation material coated on the surface of overheated coils or cores is decomposed by overheating, submicron particles having a diameter ranging from 0.001 to 1 micron are generated. These particles are diffused and float into a cooling gas circulated for cooling the coils and cores such as hydrogen, especially in a large size generator. A part of this cooling gas is extracted to the outside of the gas cooled electric machine and the overheating is detected by measuring the degree of concentration of the submicron particles contained in the cooling gas. An ion chamber particle detector is used for detecting the submicron particles. This detector is provided with an ionizing section including a source of radioactive rays and a detector section having an electric source so that the cooling gas is ionized to produce ion pairs which are generally detected as an ion current in the detector section. When the cooling gas contains submicron particles, the ion pairs are attached to the particles thereby reducing the ion current so that the concentration of the submicron particles is detected from a reduction of the ion current. An improved method of processing signals obtained by using a similar detector is disclosed in Japanese patent application No. 53-92689, which was filed July 31, 1978 by Fred Hiroshi Sawada et al as claiming the convention priority on the basis of U.S. patent application Ser. No. 820,608 filed by Fred Hiroshi Sawada et al Aug. 1, 1977 and which was published as Japanese patent laid-open No. 54-44740 on Apr. 9, 1979.
The overheating diagnostic apparatuses as disclosed by the aforementioned U.S. patent and Japanese patent application are generally useful for preventing a burning failure of the core and/or coil of a turbo-generator. However, it should be noted that these prior-art local overheating diagnostic apparatuses perform the diagnosis of local overheating through comparison of an actual value measured by the detector with an overheating reference value predetermined in terms of the magnitude of the output signal from the detector for determining whether an abnormal condition takes place inside the generator.
The inventors of this invention carried out various experimental studies on these local overheating diagnostic apparatuses to find an approach to more effective diagnosis and found that the aforementioned conventional manner of determining the overheating reference value and that of diagnosis do not necessarily achieve correct diagnosis. In other words, the magnitude of the output signal produced by the detector when dangerous local overheating takes place depends on not only the size of the rotary electric machine but on the developing condition of the overheating. Therefore, diagnosis only through the magnitude of the output signal from the detector is liable to result in misjudgement on the local overheating.